Transvenous cardiac pacemakers and ICDs using leads threaded into the right sided chambers of the heart through the venous system have evolved over the years from single chamber (one implanted lead) to dual chamber (two implanted leads); then to single and eventually dual chamber ICDs. More recently with the wider recognition of the increased incidence of heart failure secondary to right sided pacing of the heart, as well to numerous other etiologies, CRT devices have been developed with left sided chamber leads delivered transvenously to endocardial locations, and/or on through the coronary sinus to left ventricular epicardial locations, and/or with a variety of transthoracic approaches to direct epicardial or intramyocardial left ventricular or left atrial stimulation sites.
In a typical prior art dual chamber defibrillator system, there is a trifurcated ventricular lead connector, with one arm providing low voltage IS-1 pace-sense function and two arms providing high voltage DF-1 connections. An advantage of this configuration was that if one of the high voltage or low voltage components of this type of lead failed, it could be corrected relatively simply by implanting a replacement single function lead, disconnecting the failed component from the pulse generator header and plugging the replacement lead's connector into that connector cavity thereby abandoning the failed lead. However, along with the AIMD, the trifurcated connector considerably increased the mass in a patient's pectoral (or other) pocket. Furthermore, the multitude of leads provided the opportunity for cross connections at the time of initial implant. Also, as the complexity of leads increased so did the chance of lead failure and increased the difficulty and risks associated with subsequent pulse generator or lead replacement/repair surgery.
The ISO 27186 Standard for DF4 and IS4 quadripolar connector systems evolved in order to replace the mechanically and functionally complex trifurcated connector with a single lead connector encompassing multiple sequential electrodes and functions requiring only a single set screw for lead fixation and electrical activation of the pin electrode. This minimized the number and size of connector cavities (ports) in defibrillator headers. In turn, this simplified surgical implant procedures and reduced the risk of technical errors. However, lead conductor failures, particularly of IS4 and DF4 style leads have still occurred.
Failure of an implanted medical lead can occur for a variety of reasons, including dislodgement at or migration from the electrode-tissue interface, complete or partial fracture or breakage of a lead conductor, abrasion or cracking or other forms of lead insulation disruption leading to low insulation resistance and low impedance measurements. Low insulation resistance can occur between a lead conductor and body fluid or between a lead conductor and adjacent lead conductors. Other reasons for failure include an increase in lead conductor impedance, an increase of the pacing capture threshold, or just the failure to deliver appropriate, effective or optimal therapy. As defined herein, a lead conductor failure may include one or more of any of the aforementioned conditions.
When a lead fails it is not always practical to extract an IS4 or DF4 lead even if a single conductor or function has failed. The lead extraction procedure becomes particularly more difficult as the duration of implantation lengthens. Over time, the lead typically becomes adhered to tissue due to the formation of scar tissue, tissue ingrowth and the like thus requiring a more invasive procedure to be performed. On the other hand, simply abandoning a defective IS4 or DF4 lead is problematic, because abandoning the old lead and implanting a new one can lead to venous occlusion and interference with closure of the tricuspid valve leaflets etc. Further, stacked ICD leads with large surface area and high voltage coil electrodes tend to induce significant fibrous tissue reaction, binding the leads together and to the surrounding tissues making extraction procedures even more hazardous. Yet extraction may in some cases become unavoidable because of the development of endocarditis or other complications.
Incorporation of the low and high voltage contacts of an older trifurcated connector defibrillator lead into the newer single DF4 (or its low voltage IS4 counterpart) has a number of functional limitations, but physically DF4 is a great improvement as it: (1) reduces the total volume of the implantable system; (2) reduces the number of set screws required to connect the lead to the defibrillator; (3) reduces the need for tissue dissection within the pocket during replacement; (4) reduces lead-on-lead interactions within the implant site or pocket; and (5) eliminates the potential for DF-1 connectors from being reversed in the defibrillator header. However all of these mechanical and procedural advantages are essentially lost if there is a failure of one of the multiple lead conductors, insulation (and/or their associated electrodes) either through damage or failure to deliver effective therapy.
A failure of one lead conductor in a DF4 system leaves the physician with several bad choices. The physician can put the patient, themselves and their surgical team through a potentially difficult lead explant/extraction surgery and then put in a new DF4 lead. This is not without significant risk. Or, the implanting physician could throw away the still functional defibrillator pulse generator and try to obtain a custom replacement pulse generator with all the original connector cavities including DF4, plus an additional DF-1 connector cavity for a case where a high voltage shocking coil component of the multifunctional lead system has failed, or, plus an additional IS-1 connector cavity where a component of the low voltage pace sense multifunctional lead system has failed. If this type of device was obtainable the physician could then plug the partially defective DF4 lead connector into the new DF4 header connector cavity, implant a new DF-1 lead or IS-1 lead, as indicated and in parallel with the pre-existing DF4 lead system, and insert it into the new header's additional DF-1 or IS-1 connector cavity. However, the new ICD would cost over $20,000 and would need to be specific to not only the DF4 component failure at hand, but also to the specific subtype of ICD being replaced, i.e., single chamber, dual chamber or resynchronization. Further, to date no manufacturer has agreed to produce the series of at least 6 custom ICDs necessary to repair all combinations of lead malfunction and ICD subtypes. The cost of maintaining the whole range of replacement devices in inventory would also be high.
There are a number of problems with abandoned leads, including the problem of MRI RF field-induced overheating of such a lead or its distal electrode. Prior art abandoned lead components are problematic during MRI scans because they can pick up high-power RF induced energy which can lead to overheating of the lead and/or its distal electrode, which can heat up or even burn surrounding heart tissue. Implanted leads are generally less dangerous when they are connected to a pulse generator. The reason for this is that prior art pulse generators, including pacemakers and defibrillators generally have a feedthrough filter capacitor at the point of lead conductor ingress through the hermetic seal of the active implantable medical device. At high frequencies, such as for MRI RF pulsed frequencies, this EMI filter provides a low impedance path between the lead conductors and the AIMD housing which acts as an energy dissipating surface. Accordingly, in a high power MRI environment, much of the RF energy that is induced in the lead is diverted by the feedthrough capacitor where it is dissipated as a small temperature rise on the relatively large surface area of the pacemaker housing, which is usually a titanium housing. However, when a lead is abandoned, there is no place for this MRI RF energy to go other than at the distal tip electrode, which can still be in contact with biological cells. This can lead to significant overheating. For additional information regarding the danger of abandoned lead conductors, one is referred to a published paper entitled, PACEMAKER LEAD TIP HEATING IN ABANDONED AND PACEMAKER-ATTACHED LEADS AT 1.5 TESLA MRI, published in the Journal of Magnetic Resonance Imaging 33:426-431 (2011).
The safety and feasibility of MRI in patients with cardiac pacemakers is an issue of gaining significance. The effects of MRI on patients' pacemaker systems have only been analyzed retrospectively in some case reports. There are a number of papers that indicate that MRI on new generation pacemakers can be conducted up to 0.5 Tesla (T). MRI is one of medicine's most valuable diagnostic tools. MRI is, of course, extensively used for imaging, but is also used for interventional medicine (surgery). In addition, MRI is used in real time to guide ablation catheters, neurostimulator tips, deep brain probes and the like. An absolute contra-indication for pacemaker patients means that pacemaker and ICD wearers are excluded from MRI. This is particularly true of scans of the thorax and abdominal areas. Because of MRI's incredible value as a diagnostic tool for imaging organs and other body tissues, many physicians simply take the risk and go ahead and perform MRI on a pacemaker patient. The literature indicates a number of precautions that physicians should take in this case, including limiting the power of the MRI RF Pulsed field (Specific Absorption Rate—SAR level), programming the pacemaker to fixed or asynchronous pacing mode, and then careful reprogramming and evaluation of the pacemaker and patient after the procedure is complete. There have been reports of latent problems with cardiac pacemakers or other AIMDs after an MRI procedure sometimes occurring many days later. Moreover, there are a number of recent papers that indicate that the SAR level is not entirely predictive of the heating that would be found in implanted lead wires or devices. For example, for magnetic resonance imaging devices operating at the same magnetic field strength and also at the same SAR level, considerable variations have been found relative to heating of implanted lead wires. It is speculated that SAR level alone is not a good predictor of whether or not an implanted device or its associated lead wire system will overheat.
There are three types of electromagnetic fields used in an MRI unit. The first type is the main static magnetic field designated B0 which is used to align protons in body tissue. The field strength varies from 0.5 to 3.0 Tesla in most of the currently available MRI units in clinical use. Some of the newer MRI system fields can go as high as 4 to 5 Tesla. At the recent International Society for Magnetic Resonance in Medicine (ISMRM), which was held on 5 and 6 Nov. 2005, it was reported that certain research systems are going up as high as 11.7 Tesla. This is over 100,000 times the magnetic field strength of the earth. A static magnetic field can induce powerful mechanical forces and torque on any magnetic materials implanted within the patient. This would include certain components within the cardiac pacemaker itself and or lead wire systems. It is not likely (other than sudden system shut down) that the static MRI magnetic field can induce currents into the pacemaker lead wire system and hence into the pacemaker itself. It is a basic principle of physics that a magnetic field must either be time-varying as it cuts across the conductor, or the conductor itself must move within the magnetic field for currents to be induced.
The second type of field produced by magnetic resonance imaging is the pulsed RF field which is generated by the body coil or head coil. This is used to change the energy state of the protons and illicit MRI signals from tissue. The RF field is homogeneous in the central region and has two main components: (1) the magnetic field is circularly polarized in the actual plane; and (2) the electric field is related to the magnetic field by Maxwell's equations. In general, the RF field is switched on and off during measurements and usually has a frequency of 21 MHz to 64 MHz to 128 MHz depending upon the static magnetic field strength. The frequency of the RF pulse varies with the field strength of the main static field where: RF PULSED FREQUENCY in MHz=(42.56) (STATIC FIELD STRENGTH IN TESLA).
The third type of electromagnetic field is the time-varying magnetic gradient fields designated B1 which are used for spatial localization. These change their strength along different orientations and operating frequencies on the order of 1 kHz. The vectors of the magnetic field gradients in the X, Y and Z directions are produced by three sets of orthogonally positioned coils and are switched on only during the measurements. In some cases, the gradient field has been shown to elevate natural heart rhythms (heart beat). This is not completely understood, but it is a repeatable phenomenon. The gradient field is not considered by many researchers to create any other adverse effects.
It is instructive to note how voltages and EMI are induced into an implanted lead wire system. At very low frequency (VLF), voltages are induced at the input to the cardiac pacemaker as currents circulate throughout the patient's body and create voltage drops. Because of the vector displacement between the pacemaker housing and, for example, the TIP electrode, voltage drop across the resistance of body tissues may be sensed due to Ohms Law and the circulating current of the RF signal. At higher frequencies, the implanted lead wire systems actually act as antennas where currents are induced along their length. These antennas are not very efficient due to the damping effects of body tissue; however, this can often be offset by extremely high power fields (such as MRI pulsed fields) and/or body resonances. At very high frequencies (such as cellular telephone frequencies), EMI signals are induced only into the first area of the lead wire system (for example, at the header block of a cardiac pacemaker). This has to do with the wavelength of the signals involved and where they couple efficiently into the system.
Magnetic field coupling into an implanted lead wire system is based on loop areas. For example, in a cardiac pacemaker, there is a loop formed by the lead wire as it comes from the cardiac pacemaker housing to its distal TIP, for example, located in the right ventricle. The return path is through body fluid and tissue generally straight from the TIP electrode in the right ventricle back up to the pacemaker case or housing. This forms an enclosed area which can be measured from patient X-rays in square centimeters. The average loop area is 200 to 225 square centimeters. This is an average and is subject to great statistical variation. For example, in a large adult patient with an abdominal implant, the implanted loop area is much larger (greater than 450 square centimeters).
An abandoned lead (inactive lead) can heat during an MRI procedure just as an active lead would. However, the abandoned lead is not electrically coupled to an AIMD housing and is therefore not able to dissipate its energy safely into an AIMD housing away from vital body tissue. In some of the prior art, a lead cap has been used to attach to the proximal end of the abandoned lead. The abandoned lead cap can also comprise a significant amount of mass similar to the housing of an AIMD. The abandoned lead cap is then used to help divert energy from the lead itself during an MRI procedure. However, the efficiency and size of the abandoned lead cap is subject to available space limitations inside the human body. Furthermore, a plurality of abandoned lead caps may be required when more than one lead is being abandoned. This can result in too many lead caps being implanted into a patient.
Accordingly, there is a need for a solution that accommodates a variety of abandoned lead configurations without increasing the amount or size of lead caps or AIMDs placed into a particular patient. The present invention fulfills these needs and provides other related advantages.